In recent years, a polycrystalline Si TFT has been developed increasingly as a thin film element forming an integrated circuit on a glass substrate. As a method of forming a polycrystalline Si thin film, there is typically used an excimer laser method in which an amorphous Si film is first formed and then an excimer laser beam is irradiated, whereby the amorphous Si film is melted and recrystalized so as to obtain a polycrystalline Si film. As a laser annealing device used in the excimer laser method, a device in which a laser beam having the irradiation aperture of about 300 mm by 0.4 mm is scan-irradiated in several 10 μm pitches in a short axial direction thereof is marketed. With this laser annealing device, it is possible to form a polycrystalline Si film in which crystal grains of sub μm order are arranged at random. Therefore, TFTs having the mobility of about 150 cm2/Vs can be mass-produced with fine yield. Further, in order to make high-performance TFTs in the future, it is required to enlarge crystal grain diameter and position control of the crystal grain.
Techniques for enlarging the grain diameter of a polycrystalline Si film are disclosed in, for example, Japanese Patent Publication No. 2689596, Japanese Patent Application Laid-open No. 8-71780, Japanese Patent Laid-open No. 11-274095, MRS Bulletin, Vol. 21 (1996), March Edition, and the 61st National Convention of the Japan Society of Applied Physics, proceedings (2000), No. 2.
The technique for enlarging the grain diameter of a polycrystalline Si film, disclosed in Japanese Patent Publication No. 2689596, is to enlarge the grain diameter of a thin film part by using an amorphous Si film of two layers. However, in this technique for enlarging the grain diameter, there is no description or suggestion relating to the molten state of the film and the film structure other than the film thickness. Further, there is no description or suggestion relating to the position control of crystal grains.
On the other hand, there has been developed a pseudo single crystal Si TFT by forming a crystal grain of several μm equivalent to the channel length of a TFT while controlling the laser irradiating position.
For example, the MRS Bulletin, Vol. 21 (1996) mentioned above discloses a technique for forming a one-directional growth polycrystalline Si thin film formed of grain boundaries aligned in almost parallel by irradiating an extremely fine linear beam of 5 μm width in 0.75 μm pitches on an amorphous Si thin film formed in an island shape. Further, in the proceedings of the 61st National Convention of the Japan Society of Applied Physics (2000), there is disclosed a technique for forming a Si crystal grain grown to about 3 μm corresponding to the beam irradiation position by creating a laser beam having intensity cycles of μm order using a phase shift mask.
When crystal grain positions are controlled by the laser irradiation positions as described above, it is required to align the crystal grains obtained and the TFT channel region with high precision in TFT formation. Therefore, as disclosed in the proceedings of the 61st National Convention of the Japan Society of Applied Physics (2000), there is caused a need to provide an alignment mark for a stepper on a substrate, and to provide a camera for reading the mark on the laser irradiation device.
However, with such a camera, a problem of a laser irradiation device becoming complicated and larger is caused. In particular, a glass substrate for LCD is as large as 1 m square or so, currently. Therefore, if an additional chamber for reading the mark is provided besides annealing, the occupied area of the device increases significantly. Further, for aligning the substrate, θ correction is needed in addition to two axes of X and Y, so there is a need of a complicated stage enabling precise operation for fine adjustment. Such a device may cause an increase in cost and a decrease in working efficiency. Further, since reading of a substrate mark and alignment take time, throughput in the annealing step drops.
Further, when a phase shift mask is used, the mask must be closely attached substantially on the surface of amorphous Si, so Si atoms released from the surface of the amorphous Si film during laser annealing contaminate the mask. Therefore, expensive masks must be replaced frequently. This causes problems that the price of a laser annealing device as a manufacturing equipment becomes expensive and the working efficiency of the device is lowered.
An object of the present invention is to provide, for example, a thin film transistor and a method of manufacturing a polycrystalline semiconductor thin film, capable of easily realizing performances such as a low threshold voltage, high carrier mobility and a low leak current. In other words, an object of the present invention is to provide, for example, a thin film transistor having a channel region which is position-controlled and method of manufacturing a polycrystalline semiconductor thin film, without using an expensive and complicated laser irradiation device.